All high power solid-state lasers experience some distortion of the laser beam due to temperature and stress-induced changes in the refractive index of the lasing material. In the case of cylindrical geometry lasers, the non-uniform temperature distribution is radially parabolic, resulting in a thermal lensing of the beam. Although a fixed focal length spherical optic can correct this aberration at one operating point, the degree of thermal lensing generally varies in proportion to the level of optical pumping, or heat dissipated. Therefore, a variable powered optic is sometimes necessary to avoid mode instabilities that can result in poor mode/gain overlap and intra-resonator focusing of the beam, which can damage optics.
In long, thin solid-state gain elements, like rods, temperature dependent refractive index is the primary aberrating mechanism. However, in very thin gain elements, like disks, thermal bending of the lasing medium due to stress buildup is predominant although it is still largely a parabolic wavefront distortion in nature. This effect is compounded when multiple rods or disks are used serially in the resonator. A variable power optical element is therefore used for high power solid state lasers and amplifiers to correct thermal focus aberrations.
Outside of a laser cavity there often exists the need to dynamically change the shape of a laser beam wavefront. For example, aberrations in the wavefront due to temporally varying thermal effects inside or outside of the laser cavity can lead to decreased system performance unless the wavefront is dynamically corrected. In another application, it may be desirable to dynamically vary the radius of curvature of a laser beam wavefront in order to focus on a target whose range is varying with time.
A commonly employed method of adaptive optics consists of many individually controlled mirror segments. This method is characterized by a large optical surface area, necessitating the incorporation of large and expensive beam expanders and telescopes into the beam train. Other methods of adaptive optics, such as bimorph mirrors, may not operate with high wavefront quality at the high flux levels required for high power applications.
Many existing methods of dynamic wavefront control employ numerous addressable elements which may need to be addressed individually to achieve proper operation. Although a large number of individually addressable elements are necessary for control of wavefronts with rapid spatial variation, this presents a higher cost and unnecessary complication for applications in which the aberrations are low-order (spatial variation of the wavefront is slow). Other existing methods that perform only low-order wavefront correction suffer from low dynamic bandwidth or poor wavefront quality. Yet other approaches to wavefront correction of high power beams may employ active cooling or other heat removal methods, which can also add complexity and increase costs.